This invention relates to laser gauges for accurately measuring the distance to or displacement of a target surface of an object or for measuring the thickness of a web of material, such as a web that is being conveyed along a production line. More particularly the invention relates to a laser gauge which provides accurate measurements regardless of the position of the object being measured within the throat or air gap of the gauge.
Laser displacement gauges are well known in the prior art. They typically have at least one laser displacement sensor that includes an emitter which directs a laser beam onto a target surface to form a spot of light where the laser beam is incident on the target surface and a receiver that receives a two dimensional image of the spot via reflection of the laser light that is incident on the target surface. Integrated gauges mount both the emitter and the receiver in the same housing while older technology has them spaced apart in two separate housings. Each laser displacement sensor also includes a data processing unit with stored software that analyzes the image of the spot and outputs a sensed distance to the target surface.
Two laser displacement sensors are commonly used to measure the thickness of a web of material as it is conveyed along a machine for making or processing the web. In order to measure the thickness of the web, two laser displacement sensors are positioned one on each opposite side of the web and arranged in a mirror image configuration. They direct the laser light onto the web's opposite surfaces and compute the distance between those opposite surfaces, which is the web's thickness.
The lasers sensors are usually housed in a protective instrument enclosure that typically has an internal support frame and surrounding protective enclosure walls. A common enclosure for laser thickness gauges is a C-frame or O-frame that has an upper arm, a lower arm and a body that joins the arms at one or both ends of each arm. When operating, the web or other object being measured is located between the arms. Openings in the lower wall of the upper arm and in the upper wall of the lower arm allow transmission of the laser light beams onto the object being measured and transmission of light reflected from the light spots on the opposite surfaces of the object being measured to receiver of the sensor.
An inherent characteristic of such laser displacement sensors is that they have three critical distance specifications that define two critical parameters. Those parameters are reference distance and measurement range and they dictate sensor positioning requirements with respect to the object being measured in order for the measurements to be accurate. More specifically, the distance from the laser to the spot projected onto the surface of the object to be measured must be within the measurement range. When two laser sensors are used to measure thickness, the surfaces of the object being measured must be within the measurement range of both laser sensors.
FIG. 1 illustrates the distance specifications, the critical parameters and their relationship to prior art laser gauges. An upper laser sensor 10 is housed in an upper enclosure arm 11 of a C-frame enclosure 14 and a lower laser sensor 12 is housed in a lower enclosure arm 13 of the C-frame enclosure 14. The space between the enclosure arms 11 and 13 is an air gap (throat height or width) in which the object to be measured is positioned. The laser sensors 10 and 12 each project a light beam 16 in opposite directions onto the interposed object to be measured. The laser sensor 10 has a proximal (nearer) measurement limit 18 as one distance specification and a distal (farther) measurement limit 20 as another distance specification. The laser sensor 12 is oriented and positioned so that its proximal measurement limit is at limit 20 and its distal measurement limit is at limit 18. Midway between the measurement limits 18 and 20 is a reference point 22 that defines a reference distance from the laser sensors and is the preferred passline of a material being conveyed between the arms 11 and 13. Although each laser sensor has its own proximal limit and distal limit, in the thickness measurement configuration these measurement limits are preferably coincident as shown in FIG. 1.
For each laser sensor the surface onto which the laser light spot is projected must be at or beyond its proximal measurement limit and at or nearer than its distal measurement limit in order for the measurements to be accurate. These measurement limits define the measurement range within which the object to be measured must be confined. An example of these parameters is a laser gauge having a reference distance of 150 mm and a measurement range of ±40 mm from the reference point 22 (80 mm total measurement range). If the object being measured gets positioned outside of the measurement range, the measurement results in an error, or failure to read. For example, if the object is at position 24 or position 26, distance and thickness measurement is not able to be determined.
Laser thickness gauges in the prior art have an air gap that is considerably longer than the measurement range. That imposes measurement range restrictions because, if the material being measured moves too high or too low towards either arm, there will be a measurement failure. Consequently, if a company wants to install a laser gauge in an existing production line, they have to modify their existing production line to add material feeding and conveying equipment that controls the position of the material being sensed. The added equipment needs to maintain the material close to the passline and, under all conditions, within the measurement range. If a company is designing a new system, the design of the material conveyer must include equipment that similarly maintains the material within the same limits. In either case there is a need for the purchaser of a prior art laser gauge to provide rollers, guides, movable position systems, sometimes with position sensors, as a part of the conveyer apparatus that controls the motion and position (elevation, tension, speed) of a web or other object being measured through a conveyer in order to assure that the conveyer system always maintains the sheet within the measurement range. A prior art laser gauge cannot simply be mounted on a conveyer without such modification of the conveyer or consideration of the location of the passline.
It is therefore an object and purpose of the invention to provide a laser gauge that does not impose on the conveyer system any requirement that it control the position of a web, sheet or other object to be measured and therefore does not require any modification of an existing conveyer or additional equipment in a new design.